This invention relates to fogging devices and systems and, more particularly, to improvements in such devices and systems which increase operating efficiency and reduce noise. The subject matter of this Application is related to subject matter disclosed in my copending U.S. patent application Ser. No. 08/218,932, filed of even date herewith.
Ultrasonic foggers and fogging systems are known in the art for various applications, including, for example, humidity conditioning of indoor environments, combustion air intake conditioning for combustion based systems such as gas turbine systems, and recirculated flue gas fogging for boiler stack emission control systems In general, "fog" means water droplets in air that have a size of the order of 10 microns or less, are relatively unstable due to their small volume as compared to their surface area, and therefore can evaporate to dryness within a receiving airstream. The water droplets are propelled by the force of compressed air at velocities high enough to assure uniform mixing through cross flow injection into a receiving air stream. Examples of ultrasonic foggers and systems are described in the following U.S. Patents:
U.S. Pat. No. 4,042,016 PA1 U.S. Pat. No. 4,058,253 PA1 U.S. Pat. No. 4,118,945 PA1 U.S. Pat. No. 4,564,375 PA1 U.S. Pat. No. 4,667,465 PA1 U.S. Pat. No. 4,702,074 PA1 U.S. Pat. No. 4,731,990 PA1 U.S. Pat. No. 4,731,998 PA1 U.S. Pat. No. 4,773,846
Prior art fogging devices typically have a cylindrical body or nozzle having an axial bore into which a gas (typically air) and a liquid (typically water) are injected at controlled pressures. A resonator is spaced from and opposes the outlet (discharge) end of the bore. It is understood in the art that a compressed air orifice whose inlet (stagnation) pressure is substantially higher than its discharge pressure (usually atmospheric pressure for fogging applications) will, with inlet pressure exceeding three times the outlet pressure, achieve a velocity of MACH 1 in the bore of the fogger body. At and above MACH 1, a series of standing shock waves are produced which make a pressure controlled fogging system viable. The frequency of the shock waves is related to the dimensions of the bore. During the portions of these oscillation cycles when the water feed pressure is greater than the instantaneous air pressure, water displaces much of the air flow, droplets are sheared and entrained into the air path by the pressure pulse variations, and the droplets are accelerated into a MACH 1 or greater gas stream velocity. In general, the higher the gas (air) pressure with respect to liquid (water) pressure, the smaller the droplet size. Conversely, a lower applied pressure ratio yields larger droplets. Droplet size is quite important in a number of applications. The formation of a fog cloud of droplets is also substantially influenced by the geometry and spacing of the resonator that the air-water stream encounters after leaving the bore outlet, and the geometry of the fogger outer body, particularly the front surface thereof.
Although prior art foggers have performed reasonably well, certain problems exist. One such problem is objectionable noise. It has been established that noise can be reduced by increasing the water pressure, but when this is done it is also necessary to raise the air pressure in order to obtain the pressure differential to get the desired droplet size, typically a relatively small droplet size. [As first noted above, a larger pressure differential generally produces a smaller droplet size.] However, an increase in air pressure, in addition to increasing noise and compressed air cost, can result in undesirable turbulence in front of the fogger body, and interfere with the efficient and smooth entrainment of air which is needed to effect the desirable evaporation to dryness. Also, it has been observed that prior art foggers have tended toward a pulsating type of flow that can result in inefficient fog generation and in more compressed air than necessary being used to generate the fog.
It is among the objects of the present invention to address problems of prior art fogger operation and to generally improve fogger efficiency.